This invention relates generally to retractable lanyards, and more particularly to a mechanical clutch locking mechanism for a retractable lanyard that is configured to further ensure that the fall arrest lanyard will not fail to lock in a fall situation even if the locking components become fouled or frozen in place.
Self-retracting lanyards (retractables) are safety devices that are designed to reduce the risks of a fall for an individual who is working at what would otherwise be dangerous or deadly heights. Each retractable comprises a cable, known as a lifeline, that is held in the retractable on a reel or drum. When the lifeline is pulled from the retractable at a relatively slow rate, such as when the user is moving about but not falling, the retractable clutch lock mechanism allows the reel or drum to rotate to unwind and the lifeline to extend from the retractable housing. However, when the lifeline is pulled from the retractable at a very rapid rate such as when a user is falling, a clutch or shock absorber or other similar clutch lock mechanism in, or associated with, the retractable reel or drum will automatically engage and slow and/or stop the reel or drum from rotating to slow or stop the unwinding of the lifeline. This halts the individual's fall after only a very brief interval.
One such retractable has an internal clutch system in which a pawl plate has a stack of friction discs on each side of a clutch plate to which the drum is connected and which can apply up to approximately 3000 pounds per square inch of compressive force to each side of the plate. This creates normal forces on friction pads that can slip under load, thus softly stopping the release of the lifeline.
Lock mechanisms for retractables also can be made in which the mechanisms that sense a fall has occurred can be activated either by sensing displacement, velocity, or acceleration. Most retractable lanyards lock up using the principal of a pivoting pawl which moves upon rotation of the drum to engage a locking wheel (commonly referred to as a sperrad) that is connected to the clutch plate.
In locking mechanisms that use “displacement,” a rotating cam engages the pawl. As the drum rotates, the cam raises and lowers the pawl into an engagement zone with the sperrad. If the drum turns fast enough (as in a fall), the cam will rotate so quickly that it will cause the pawl to leave the cam surface and rise sufficiently to engage a stationary sperrad. This stops the rotation of the drum, and causes the clutch plate to begin rotating between the friction disks, allowing for a slow deceleration and eventual stopping of the decent of the falling worker. Clutch lock mechanisms are usually designed to limit the arrest forces to 900 lbs. average (or less).
In locking mechanisms that use “velocity” to determine pawl activation, the pawl is pivotally mounted on the drum. The pawl will have a pointed end and possibly a counterweighted end. The counterweighted end may be lighter or heavier than the pointed end depending on whether the pawl swings outwardly or inwardly to engage the sperrad. The pawl is restrained from engagement with the sperrad at low rotational speed of the drum by use of a spring which keeps the pointed end of the pawl out of the sperrad's engagement zone until the drum has reached a fall arrest velocity. When that rotational velocity is reached, the pawl will swing out into the engagement zone and engage the tip of the sperrad to create lockup, and thus stop rotation of the drum.
In most retractables, when a pawl is driven using velocity (i.e., “centrifugal forces”) to engage a sperrad, under certain circumstances the pawl may not engage the sperrad, such as if the pawl is fowled with debris, or frozen (either due to ice or thermal contraction). This results in a situation in which the lanyard locking components may become frozen in a non-engaging position that can allow the lifeline to freely unwind from the lanyard drum without stopping. Should a user be attached to a lanyard in this condition and fall, the lifeline may continue to discharge to its full length, thereby causing serious injury or even death to the user. This condition can occur regardless of whether the lockup mechanisms use displacement, velocity, or acceleration to activate the pawl. It is only possible to substantially guarantee lockup between the pawl and the sperrad in mechanisms that use displacement (i.e., cam driven pawls) to drive the pawl tip into an engagement zone that is beyond the sperrad tip diameter. The spring then is used to pull the pawl tip out of the engagement zone just prior to passing the sperrad tip. This mechanism of using a cam driven pawl in which the cam pushes the pawl tip into an engagement zone creates a pawl mechanism in which nearly any failure of pawl rotational freedom guarantees lockup.